This invention relates to a method and apparatus for displaying high resolution binary image data on a CRT having a resolution which is lower than the resolution of the binary image data with a minimal loss of visual information.
At the present time, documents are scanned along a scanning line to produce a predetermined resolution or number of pixels (picture elements) per inch. For example, the resolution may range from about 100 to about 300 pixels per inch, with about 200 pixels per inch being used by certain industry groups, like facsimile equipment manufacturers, for example. Display screens to display a pixel density of 300 per inch are extremely expensive if they are available at all.
While the image data for documents is produced in the densities mentioned in the previous paragraph, it is difficult to obtain low-cost, high density viewing screens. For example, high resolution viewing screens for CRTs have densities of 75, 100, or as high as 150 pixels per inch. In order to display (at normal size) an image developed at 200 pixels per inch on a screen having a resolution of 100 pixels per inch, it is necessary to perform some compaction of the image. For example, a 100 pixel per inch screen requires a 2-to-l compaction for image data which was developed at 200 pixels per inch.
One simple option for compaction is referred to as pixel dropping. This technique entails removing rows and columns of pixels from the image data to fit the appropriate size for the display screen; however, a problem with this technique is that narrow horizontal or vertical lines can be lost in the process, and edge aliasing (raggedness) is produced. Other elaborate schemes of this type which can preserve horizontal and vertical lines are available; however, these schemes tend to create unwanted "artifacts" on the displayed image.
Another way of handling the mismatch between the density of the image data and the resolution of the associated display screen is to display as much of the image as possible without dropping any pixels. In this situation, the image displayed is larger than the original, and a correspondingly smaller fraction of the original image area is visible on the display. In this case, the edges of the image will appear more ragged due to the magnification of the image.
In certain applications, having less than the entire image on a display screen presents an unsatisfactory solution. For example, in the processing of financial documents, like checks, it is necessary to look at the entire image of a check on a display screen in order to verify the monetary amount and other data on the check as part of normal processing.